Infrared motion detection signal sampler

ABSTRACT

The digital sampler circuit processes an output signal from a high gain analog amplifier having as input an infrared motion detector output signal and a negative low pass filtered feedback signal. The amplifier output signal is a substantially square pulse signal having an irregular frequency and duty cycle and has an average duty cycle indicative of a DC level and a slow change in the DC level of the detector output signal. The sampler circuit includes a detector circuit for detecting the amplifier output signal and discriminating at regular intervals a high/low state of the output signal, and generates a high/low feedback signal corresponding to the high/low state of the output signal detected. The high/low state of the output signal is analyzed over a predetermined number of the intervals to obtain a ratio value for producing an output digital signal sample value. The digital sample value is a measure of the DC level and the change in the DC level of the detector output signal. The digital sample is accurate and requires a single comparator and relatively simple circuitry.

FIELD OF THE INVENTION

The present invention relates to an infrared motion detection digitalsignal sampler circuit. More specifically, the invention relates to sucha sampler circuit integrated in the feedback loop of a high gain analogamplifier for processing an output signal of the amplifier. Theamplifier has as input an infrared motion detector output signal and anegative feedback signal. The amplifier output signal is a substantiallysaturated, substantially square pulse signal having an irregularfrequency and duty cycle, and an average duty cycle of the amplifieroutput signal is indicative of a DC level and a change in the DC levelof the motion detector output signal.

BACKGROUND OF THE INVENTION

Passive infrared motion detector are well known in the art. Suchdetectors typically include a small housing, a lens for directinginfrared light from a zone to be monitored onto an infrared sensorelement. The most common type of sensor element is a pyroelectric sensorwhich generates a small but detectable electrical voltage in response tochanges in infrared radiation impinging on it. Due to the lens of theknown detectors, motion of a person into or from the monitored zonechanges the intensity of infrared light striking the sensor and producesa voltage variation. A discriminator circuit monitors the sensor outputsignal and generates an alarm signal provided that certaincharacteristics are present in the sensor output signal. Suchcharacteristics may be energy, amplitude peaks, number of oscillations,etc.

An example of the prior art detectors is found in U.S. Pat. No.5,077,549, co-invented by the inventor of the present invention. It isknown from this reference to provide an infrared motion detector circuitwhich uses a digital logic circuit to discriminate the sensor outputsignal. The sensor signal amplitude is converted by a voltage controlledoscillator (VCO) to a pulse frequency, and a counter circuit countspulses to detect the sensor signal's energy and generate the alarmsignal.

In the past, the use of a microprocessor along with an analog-to-digitalconverter (ADC) has been prohibitively expensive for use in acommercially competitive motion detector discriminator circuit. Intoday's market, various commercially available microcontrollers ormicroprocessors are inexpensive enough to competitive with standard“hard wired” integrated circuit technology. While the use of amicroprocessor improves the flexibility and possible quality of thesignal analysis and discrimination, it still requires circuitry forgenerating a digital signal from the sensor's analog output, such as ahigh gain bandpass amplifier and an ADC, which represents a significantpart of the present cost of the detector circuitry.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an infrared motiondetection digital sampler circuit for processing an infrared sensoroutput signal to generate digital samples without requiring ananalog-to-digital converter. It is a further object of the invention toprovide such a digital sampler circuit, integrated in the feedback loopof the bandpass amplifier, which can rely on a microprocessor'scapabilities to obtain the desired digital samples by reading the outputsignal of a high gain analog amplifier having as input the infraredmotion detector output signal and a low pass filtered negative feedbacksignal and providing a control negative feedback signal to the amplifierinput. It is a further object of the invention to provide a digitalsignal processor for an infrared motion detector utilizing amicroprocessor and a single operational amplifier.

According to the invention, there is provided an infrared motiondetection digital sampler circuit for processing an output signal from ahigh gain analog amplifier having an input an infrared motion detectoroutput signal and a negative low pass filtered feedback signal, theamplifier output signal being a substantially saturated, substantiallysquare pulse signal having an irregular frequency and duty cycle, anaverage duty cycle of which is indicative of a DC level and a slowchange in the DC level of the detector output signal. The detectioncircuit comprises means for detecting the output signal anddiscriminating at substantially regular intervals a high/low state ofthe output signal; means for generating a high/low feedback signalcorresponding to the high/low state of the output signal detected; meansfor analyzing the high/low state of the output signal detected over anumber of the regular intervals to obtain a high/low ratio value and foroutputting as a digital signal sample value this ratio value. In thisway, the digital signal sample value is a measure of the DC level andthe slow change in the DC level of the detector output signal in thedesired frequency range of the detector, approximately 0.1 Hz to 10 Hz.Preferably, the analyzing means comprise a digital counter.Alternatively, the output signal can be integrated to obtain an analogvalue representing the signal sample.

The invention further provides an infrared motion detector signalprocessing circuit comprising a high gain analog amplifier having aninput an infrared motion detector signal and a negative low passfiltered feedback signal, and producing an output signal which is asubstantially saturated, substantially square pulse signal having anirregular frequency and duty cycle. The average duty cycle of the squarepulse signal is indicative of a DC level and a slow change in the DClevel of the motion detector signal. The motion detection circuitfurther comprises means for detecting the output signal anddiscriminating at substantially regular intervals a high/low state ofthe output signal, means for generating a high/low feedback signalcorresponding to the high/low state of the output signal detected, andmeans for analyzing the high/low state of the output signal over anumber of the intervals to obtain a high/low ratio value and foroutputting as a digital signal sample value the ratio value. In thisway, the digital signal sample value is a measure of the DC level andthe slow change in the DC level of the detector output signal in thedesired frequency range of the detector, approximately 0.1 Hz to 10 Hz.

According to a second broad aspect of the present invention, there isprovided a method for infrared motion detection comprising: providing amicroprocessor; providing a motion detector; providing analog circuitry;superposing an ac signal output from said motion detector onto a DCsignal to create an input signal; feeding said input signal into saidanalog circuitry such that a generated output varies between two logiclevels; sampling a voltage level of the output in order to obtaindigital signal samples; and counting said digital signal samples usingsaid microprocessor.

According to a third broad aspect of the present invention, there isprovided a method for infrared motion detection comprising: providing amicroprocessor; providing a motion detector; providing analog circuitry;superposing an ac signal output from said motion detector onto a DCsignal to create an input signal; feeding said input signal into saidanalog circuitry such that a generated output varies between two logiclevels and sampling a voltage level of the output in order to obtaindigital signal samples; and analyzing said digital signal samples usingsaid microprocessor.

BRIEF DESCRIPTION OF THE DRAWING

The invention will be better understood by way of the following detaileddescription of a preferred embodiment of the invention with reference tothe appended drawing in which:

FIG. 1 is a schematic block diagram of the infrared motion detectionampler circuit according to the preferred embodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

As shown in FIG. 1, the infrared motion detector 10 according to thepreferred embodiment comprises a pyroelectric sensor 12 connected to thepositive input of a high gain amplifier 14. The sensor is a HeimannLHI958 pyroelectric sensor. The sensor output signal is filtered by ahigh pass filter 21 and then a low pass filter 22 and a DC biasreference level 23 is added to the filtered output signal before beingconnected to amplifier 14. High pass filter 21 has a threshold of 0.2Hz, and low pass filter 22 has a threshold of 7.0 Hz. The high passfilter is a first order filter, and the low pass filter is a secondorder filter to eliminate alias. The DC bias level generated by circuit23 is 1.0V. The gain of amplifier 14 is typically higher than 100000.

The feedback circuit comprises a pulse detector 24 which detects theoutput of analog amplifier 14 and discriminates the voltage into a lowor high signal. The HI/LO digital signal is repeated by a feedback pulsegenerator 27 and fed back to amplifier 14 via a temperature compensator28 and an integrator 18. Integrator 18 in the preferred embodiment has aresistance of 1 MΩ and a capacitance of 100 μF. The integrator 18 mayalso comprise a low pass filter which filters out the high frequency ofthe amplifier output and passes through the low frequency voltage levelpresent in the amplifier output. The digital signal level is thusreduced below the minimum detectable input signal, i.e. below 1 mV.

The temperature compensation circuit 28 includes a thermistor in avoltage divider arrangement to adjust the feedback voltage and gain inaccordance with temperature in order to increase sensitivity as theambient temperature increases. Increased sensitivity is required athigher temperatures, since the difference between infrared radiationlevels emitted by the ambient environment and a person moving through adetection zone decreases with higher ambient temperatures. As a resultof temperature compensation, the duty cycle in the pulse train outputfrom the amplifier 14 increases with temperature. The DC level of thefeedback voltage fed to amplifier 14 thus changes with temperature as aresult of circuit 28. Changes in temperature thus cause a change in thebase duty cycle in the amplifier output. By monitoring this base dutycycle, the temperature compensation circuit 28 can also be used toobtain a temperature reading which can be used to change how the digitalsignal samples are interpreted to generate an alarm signal.

The amplifier 14 is arranged as a non-inverting mode differentialamplifier having an output which is a substantially saturated,substantially square pulse signal. Due to the high gain of the amplifier14, a small difference between the filtered sensor signal and theintegrated feedback signal produces a saturated maximum output voltagefrom the amplifier 14, or a saturated zero output voltage depending onthe polarity of the difference. The maximum voltage of the amplifier 14is set to 5V. The output of amplifier 14 has an average duty cycleproportional to a sum of a change in the DC level at the positive inputand the DC level itself. The feedback circuit is designed so as toprovide a reliable measurement of millivolt changes in the sensoroutput. The square pulse signal output of amplifier 14 has an irregularfrequency and pulse width. The frequency of the square pulse output ofamplifier 14 depends on the circuit loop response and also on thefeedback pulse period, but the frequency is not important to obtainingsamples. The maximum HI to LO to HI speed can be very high and dependson the loop speed of the circuit components, as well as the rate atwhich the amplifier output is detected and fed back by generator 27.

The amplifier 14 according to the preferred embodiment is adifferentiator amplifier operating in a non-inverting mode, however, aninverting mode differentiator amplifier could also be used. In the caseof an inverting mode amplifier, the sensor signal is connected to thenegative input along with the feedback with a suitable resistanceconnected between the integrator and the negative input, and a DCreference voltage is connected to the positive input of the amplifier.

The pulse detector 16 samples at a substantially regular frequency ofabout 16 kHz. Detector 16 decides whether the analog output of theamplifier 14 is HI (i.e. 5V) or LO (i.e. 0V) and outputs a bit signal.In the preferred embodiment, the pulse detector is part of amicroprocessor and detection of the amplifier 14 output is done byreading a digital input signal pin on a regular basis controlled bymicroprocessor interrupt. Due to the finite gain of amplifier 14, anon-saturated output (i.e. not 5V or 0V) will occur whenever thefeedback voltage is very close to the positive input voltage. Suchintermediate voltage levels will be randomly interpreted by thediscriminator in pulse detector 24, and such random or erroneousinterpretation will lead to variations in the feedback and contribute tothe pulse train output of amplifier 14 having an irregular, randomcharacter.

The counter 25 counts the number of HI pulses (‘1’ bits) in 255 cycles.The result of the counter gives a single sample which can be stored inone byte of data. The counting and data storage is done in software bythe microprocessor in the preferred embodiment. This yields about 62samples each second, which is more than adequate for analyzing themotion sensor output signal of less than 15 Hz. The digital signalanalysis is done in a digital signal processor 26, which in thepreferred embodiment is provided by software in the microprocessor.

Since the feedback speed is controlled by the digital sampling andfeedback signal generation speed, the output signal from the amplifierwill have a maximum switching frequency lower than the feedback signalgeneration speed, and thus analysis of the high/low state of theamplifier output to obtain the digital signal sample value can be doneefficiently at the rate of the feedback signal generation.

The digital samples obtained using the present invention represent a sumof the DC signal and a change in the DC signal input to the amplifier14. This represents the ratio of the HI to LO state of the amplifieroutput. As an example, a constant duty cycle of 25% may be present whenthe amplifier input is constant, i.e. the sensor output is stable at 0Vand the DC bias is at its predetermined level. The single byte samplevalue may be around 64 for a 25% duty cycle. A small change in sensorvoltage, namely +2 mV, would only result in a small change in theconstant duty cycle, namely it may rise to 25.05%. Thus, the digitalsample value having a magnitude of 255 may only change by one digit.However, during the change in input voltage, the duty cycle representsthe base DC level plus the rate of change (slope) of the sensor output.A 2 mV change over 0.2 s is a 10 mV/s rate of change which may cause theduty cycle to increase by 5 percentiles to 30% over the 0.2 s period.Likewise a drop in sensor output by 2 mV in 0.2 s would cause the dutycycle to drop by 5 percentiles to 20% during the voltage decline. Thisresults in transition duty cycles that produce easily detectable changesin the digital sample values, e.g. about 15 units.

It will be appreciated that each digital sample results from a simplecount or sum of the detected amplifier output voltage level over asufficiently long period of time to compensate for the random variationsin the pulse train and obtain a stable accurate measurement of thechange in the output signal from sensor 12.

The analog amplifier may be a differential amplifier, a comparator or asingle input amplifier. The analog amplifier can be an externalcomponent or an internal integral part of a microcontroller.

Although the invention has been described above with reference to apreferred embodiment, it is to be understood that the above descriptionis intended merely to illustrate the invention and not to limit thescope of the invention as defined in the appended claims.

1. An infrared motion detection digital sampler circuit for processingan output signal from a high gain analog amplifier having as input aninfrared motion detector output signal and a negative, low pass filteredfeedback signal, said output signal being a substantially saturated andsubstantially square pulse signal having an irregular frequency and dutycycle, an average duty cycle which is indicative of a DC level and aslow change in said DC level of said detector output signal in a desiredfrequency range of said motion detector output signal, said detectioncircuit comprising: means for detecting said output signal anddiscriminating at substantially regular intervals a high/low state ofsaid output signal; means for generating a high/low feedback signalcorresponding to said high/low state of said output signal detected;means for analyzing said high/low state of said output signal over anumber of said intervals to obtain a high/low ratio value and foroutputting as a signal sample value said ratio value, whereby saidsignal sample value is a measure of said DC level and said change insaid DC level of said detector output signal.
 2. The circuit as claimedin claim 1, wherein said analyzing means comprises a counter circuitsampling said high/low state at said regular intervals, said ratio valuebeing a digital counter value.
 3. The circuit as claimed in claim 1,wherein said detector output signal is connected to a second order lowpass filter, whereby alias is substantially eliminated.
 4. The circuitas claimed in claim 1, wherein said feedback signal is connected to atemperature compensation circuit.
 5. The circuit as claimed in claim 2,wherein said detecting means comprise a microprocessor, said intervalsbeing determined by interrupts, said analyzing means being provided bysaid microprocessor.
 6. The circuit as claimed in claim 5, wherein saidpredetermined number of intervals is 256 and said signal sample value isstored as one byte.
 7. The circuit as claimed in claim 6, wherein afrequency of output of said signal sample value is greater than 30 Hz.8. The circuit as claimed in claim 1, further comprising filter meansfor filtering said motion detector output signal, said filter meanscomprising a high pass filter, a low pass filter and a DC bias.
 9. Thecircuit as claimed in claim 1, wherein said negative feedback signal ispassed through an integrator for providing flat frequency response insaid desired frequency range of said motion detector output signal. 10.An infrared motion detector signal processing circuit comprising: a highgain analog amplifier having as input an infrared motion detector outputsignal and a negative low pass filtered feedback signal, and producingan output signal, said output signal being a substantially saturated,substantially square pulse signal having an irregular frequency and dutycycle, an average duty cycle which is indicative of a DC level and aslow change in said DC level of said motion detector signal in a desiredfrequency range of said motion detector signal; means for detecting saidoutput signal and discriminating at substantially regular intervals ahigh/low state of said output signal; means for generating a high/lowfeedback signal corresponding to said high/low state of said outputsignal detected; means for analyzing said high/low state of said outputsignal over a number of said intervals to obtain a high/low ratio valueand for outputting as a digital signal sample value said ratio value,whereby said digital signal sample value is a measure of said DC leveland said change in said DC level of said detector output signal.
 11. Thecircuit as claimed in claim 10, wherein said analyzing means comprises acounter circuit sampling said high/low state at said regular intervals,said ratio value being a digital counter value.
 12. The circuit asclaimed in claim 10, wherein said detector output signal is connected toa second order low pass filter, whereby alias is substantiallyeliminated.
 13. The circuit as claimed in claim 10, wherein saidfeedback signal is connected to a temperature compensation circuit. 14.The circuit as claimed in claim 11, wherein said detecting meanscomprise a microprocessor, said intervals being determined byinterrupts, said analyzing means being provided by said microprocessor.15. The circuit as claimed in claim 14, wherein said predeterminednumber of intervals is 256 and said signal sample value is stored as onebyte.
 16. The circuit as claimed in claim 15, wherein a frequency ofoutput of said signal sample value is greater than 30 Hz.
 17. Thecircuit as claimed in claim 10, further comprising filter means forfiltering said motion detector output signal, said filter meanscomprising a high pass filter, a low pass filter and a DC bias.
 18. Thecircuit as claimed in claim 10, wherein said negative feedback signal ispassed through an integrator for providing a flat frequency response insaid desired frequency range of said motion detector signal.
 19. Amethod for infrared motion detection comprising: providing amicroprocessor; providing a motion detector; providing analog circuitry;superposing an ac signal output from said motion detector onto a DCsignal to create an input signal; feeding said input signal into saidanalog circuitry such that a generated output varies between two logiclevels; sampling a voltage level of the output in order to obtaindigital signal samples; and counting said digital signal samples usingsaid microprocessor.
 20. A method as claimed in claim 19, wherein saidproviding analog circuitry further comprises providing an amplifier toinput an analog signal and a second signal and output a digital signal.21. A method as claimed in claim 20, wherein said second signal is afeedback signal output from said amplifier.
 22. A method as claimed inclaim 21, further comprising integrating said feedback signal to providea flat frequency response in a desired frequency range of said motiondetector output signal.
 23. A method as claimed in claim 21, furthercomprising providing a temperature compensation circuit for saidfeedback signal.
 24. A method as claimed in claim 19, further comprisingfiltering said ac signal from said motion detector to substantiallyeliminate aliasing.
 25. A method as claimed in claim 19, wherein saidcounting further comprises counting a predetermined number of intervalsdetermined by interrupts.
 26. A method as claimed in claim 25, whereinsaid predetermined number of intervals is 256, and a signal sample valueis stored as one byte.
 27. A method for infrared motion detectioncomprising: providing a microprocessor; providing a motion detector;providing analog circuitry; superposing an ac signal output from saidmotion detector onto a DC signal to create an input signal; feeding saidinput signal into said analog circuitry such that a generated outputvaries between two logic levels; sampling a voltage level of the outputin order to obtain digital signal samples; and analyzing said digitalsignal samples using said microprocessor.